Only one? Hmm...
Well, I would want something easily concealable, easy to carry, and quick to draw. Since I'm not legally allowed SBR's or machine-guns, that likely means a pistol.
Next, I would want something semi-automatic and in a common caliber. That means I'm pretty much looking at 9mm, .40 S&W, or .45 acp.
Out of the three, I'd choose .40 S&W since it essentially has the best balance of trajectory, energy, bullet weight, and magazine capacity.
Finally, since I might have to hunt with it, I'd want a longer barrel and sight picture.
Put all that together and it sounds like a Glock 24 or 35 comes pretty close.